The present invention relates to a mobile radiophone, and more particularly to a radio-frequency (RF) power control circuit of a transmitter in a mobile radiophone.
Generally, the mobile radiophone includes a vehicle radiophone (hereinafter referred to as car phone) and portable hand-held phone (hereinafter referred to as HHP). The mobile radio communication system for operating the car phone and HHP employs cellular radio communication method. The cellular radio communication method is to divide the whole service area into a plurality of sub-areas called cells and to employ a different frequency for each of the cells, in which if there is no frequency interference between the cells, the used frequency band is employed again to increase the communication efficiency and capability and to improve the communication quality. The known cellular communication method may be represented by the AMPS (Advanced Mobile Phone System) developed by the Bell Laboratory and ARTS (American Radio Telephone Service) developed by AJ & J.
A mobile radio communication system includes a mobile telephone switching office, cell sites, and mobile radiophones. The mobile telephone switching office communicates with the cell site on wire, while the cell site communication with the mobile radiophone is wireless. The mobile telephone switching office interconnects the wire telephone line network with the mobile radio communication system, and controls the channels allotted for the cell sites. The cell site controls various signals to deal with the wire telephone network in cooperation with the mobile telephone switching office, monitors the signal intensity of mobile radiophones within its own cell area to provide the information of the signal intensity to the mobile telephone switching office, and transfers to mobile radiophones the information received from the mobile telephone switching office. Thus, telephone subscribers connected with the mobile telephone switching office can communicate with mobile radiophones. If the power of the RF signal of the mobile radiophone near the cell site is increased, the communication sensitivity to the mobile radiophone distant from the cell site decreases. Hence, the cell site monitors the signal sensitivity of the mobile radiophone within its cell in order to adjust the signal sensitivity. The mobile radio telephone system prescribes the effective radiation power (ERP) according to the classes of the mobile radiophones in order to improve communication sensitivity, as shown in the following table 1.
TABLE 1 ______________________________________ EIA Specification (Power Output and Power Control) ERP (dBw) Power Level according to Power Class (PL) MAC I II III ______________________________________ PL0 000 6 2 -2 PL1 001 2 2 -2 PL2 010 -2 -2 -2 PL3 011 -6 -6 -6 PL4 100 -10 -10 -10 PL5 101 -14 -14 -14 PL6 110 -18 -18 -18 PL7 111 -22 -22 -22 ______________________________________
Therefore, monitoring the signal sensitivity of the mobile radiophone, the cell site outputs the mobile attenuation code (MAC) signal of three bits in order to control the signal intensity, in response thereto. Receiving the MAC signal, the mobile radiophone radiates the RF signal for the Power Level of a given class. Power class is determined according to the classes of the mobile radiophone as shown in Table 1, for example, the car phone classified into class I, HHP classified into class III. Thus, the total number of the power levels of the car phone is 8, and each level is reduced by 4-dB starting from the highest level (PL0). On the other hand, the total number of the power levels of HHP is 6 (PL0-PL2 is common), and each level is reduced by 4-dB starting from the highest level (PL0-PL2).
Now, the conventional RF power control mechanism will be described with reference to FIG. 1. At first, the logic controller of the mobile radiophone, receiving the MAC signal from the cell site, generates a logic control signal. The logic control signals a,b and c have 8 states of 000-111 in binary code. According to the signals of the 8 states, analog multiplexer 11 sets the RF power levels PL0-PL7 as shown in Table 2.
TABLE 2 ______________________________________ a b c Output ______________________________________ 0 0 0 PL0 0 0 1 PL1 0 1 0 PL2 0 1 1 PL3 1 0 0 PL4 1 0 1 PL5 1 1 0 PL6 1 1 1 PL7 ______________________________________
After the power level specification (RF output measurement) as shown in Table is determined 1 variable resistors are adjusted to satisfy the power level specification, the power levels (PL0-PL7) as shown in Table 2 are set. For example, when setting the state of PL0, the variable resistor VR11 is adjusted to satisfy the power level specification (RF output measurement) of PL0. Then, because the state of PL1 is subordinated to the state of PL0, it may be set by resistors R12, R13. In this way, PL2, PL4 and PL6 are set and the variable resistors VR12, VR13, VR14 are then adjusted to meet the power level specification. Thereafter, it is confirmed that PL3, PL5, PL7 exist within their corresponding power level specification.
If the power levels PL1-PL7 are set as described above, the analog multiplexer 11 produces output voltage Vo for the given power level according to the logic control signal generated by the logic controller. The range of the output voltage Vo is determined by each element of automatic gain control (AGC) circuit 14, particularly by the differential driving operation of three-to-four-stage power amplifier 13 and the characteristics of the power amplifier 13. In the case of a mobile radiophone such as car phone and HHP, the output voltage Vo is about 0.5-4 volts.
The output voltage Vo of the analog multiplexer 11 drives the power amplifier 13 through a comparing amplifier 12 within AGC circuit 14. The power amplifier 13 amplifies the modulated signal (data+RF carrier-wave) received from the modulator of the mobile radiophone according to the output of the comparing amplifier 12 so as to output RF signal through antenna. Here, the output of the power amplifier 13 is fed back through a high-frequency coupling circuit, a rectifying circuit and a voltage dividing circuit within the AGC circuit 14 to one input of the comparing amplifier 12 as a comparing voltage Vf. Thus, the gain of the given voltage level chosen by the analog multiplexer 11 is automatically compensated, so that a constant voltage may be supplied to the power amplifier 13.
However, according to such a conventional RF power control method, in order to set each of six to eight power levels to the given power level specification, three to four variable resistors are adjusted and their subordinate power levels are set by resistors, thereby consuming. Further the power levels are set manually one by one. Therefore, if the value of a variable resistor deviates, the associated power level as well as the subordinate power level deviates from the specification, thereby resulting in difficulty in setting the power level. Especially, precise adjustment of the variable resistor is hardly achieved in the low power level, and vibrations and impacts caused by moving of the mobile radiophone easily cause change of the variable resistors, so that the power levels deviate from the desired specification, thereby impairing the reliability of the system.